In recent years, from a viewpoint of saving energy, it has been desired to reduce watt-loss in electrical steel sheets. As a method for reducing the watt-loss, a subdivision of magnetic domains by laser irradiation has already been disclosed in Japanese examined Patent Publication (Kokoku) No. 58-26405. The reduction of watt-loss by this method is due to strains introduced by laser irradiation. Therefore, the method can be applied to a laminated-core type transformer wherein no stress-relief annealing is necessary, but cannot be applied to a wound-core type transformer wherein stress-relief annealing is necessary. Also, Japanese Unexamined Patent Publication (Kokai) No. 59-100222 discloses a method wherein a steel sheet subjected to secondary recrystallization annealing is locally heat-treated and annealed at a temperature of 800.degree. C. or higher, whereby grain boundaries are artificially introduced. In this method, the reduction of watt-loss value is achieved by the subdivision of magnetic domains by the artificial grain boundaries introduced into the steel sheet. The watt-loss reduction effect does not disappear even upon stress-relief annealing, because the steel sheet is annealed at a temperature of 800.degree. C. or higher. However, the disclosed examples indicate it is difficult to obtain a watt-loss comparable with that in the above-mentioned method for reducing the watt-loss value by laser irradiation.